There are several ways you can get teh right market price based on car year, kms, color and condition.
Take the Min / Max average of the dubizzle or classifieds site ads.
Reduce 10% for every year, for the number of year you own the car, if it's japanese. For UK, European brand reduce 15%.
If car is brand new reduce 20% for first year and then 10% ech year
Get the evaluation done from third party car dealers who doesnt buy car, but gives a fair evaluation